Shape memory alloy (SMA) elements exhibit a temperature related shape transformation corresponding to an inherent bi-stable crystalline structure. This characteristic has been used to advantage in various control devices, with the temperature of the SMA element being controlled by electric heating. For example, it has been proposed to use an SMA element to move an armature for opening and closing a fluid valve, such as described in the U.S. Pat. No. 4,973,024, issued on Nov. 27, 1990, and in the U.S. Pat. No. 5,325,880, issued on Jul. 5, 1994. The control of these and other SMA devices may be either open-loop (i.e., based on experimentally determined characteristics of the device) or closed-loop (i.e., based on a feedback signal related to the parameter being controlled). Additionally, or alternatively, the resistance of the SMA element can be monitored for feedback purposes, as mentioned in the above-referenced U.S. Pat. No. 5,325,880, since the SMA material exhibits a characteristic change in resistivity during its shape transformation.
The application of SMA materials to a precision metering device such as an automotive fuel injector presents a significant challenge in terms of control since the performance of the device tends to vary substantially with ambient temperature. Although the ambient temperature variation can be mitigated to a degree in such an application by cooling the SMA element with circulation of excess fuel through the injector, the fuel temperature still varies significantly depending on ambient conditions. The control techniques alluded to above are not satisfactory since the device characteristics change with temperature and the controlled parameter cannot be directly measured for feedback purposes. Accordingly, what is needed is a control that causes the valve to open quickly and fully, and then close quickly, with minimum variability.